1. Field of the Invention
The present invention relates to a printed-wiring substrate having lead pins. More particularly, the invention relates to a printed-wiring substrate which is formed primarily from an insulating material, such as resin or ceramic, and has a number of pin-bonding portions (electrodes) formed on a main surface thereof. Lead pins (input/output terminals) are brazed to the corresponding pin-bonding portions; for example, to a printed-wiring substrate of a PGA (pin grid array) type (IC package), on which an electronic component, such as a semiconductor integrated circuit device (IC), is mounted in a sealed condition.
2. Description of the Related Art
A PGA type printed-wiring substrate (hereinafter, also referred to as a substrate) has a number of bonding portions (electrodes), each of which assumes the form of a pad, formed on one main surface for bonding to an IC, as well as a number of lead pins (hereinafter, also referred to as pins) provided on the other main surface for insertion into a socket provided on a motherboard. Each lead pin assumes the form of, for example, a nail and is brazed to a substrate such that a flange (a portion of the pin having a diameter greater than that of a shaft portion of the pin) located at an end portion of the pin abuts a pin-bonding portion of the substrate.
The above-mentioned pin-bonding structure is disclosed in Japanese Utility Model Application Laid-Open (kokai) No. 60-106375. Specifically, a convex portion is formed on the bonding surface of the flange located at an end portion (head portion) of the nail-shaped pin. The bonding surface of the flange faces a pin-bonding portion of the substrate. The pin is brazed to the substrate such that the convex portion abuts the pin-bonding portion. In the case of brazing a pin having a flange whose end portion serving as a bonding surface is flat, the amount of brazing filler metal present between the flange and a pin-bonding portion is small, resulting in small bonded area. By contrast, a convex portion allows for a larger amount therebetween. Since the large amount of brazing filler metal effects a stress-absorbing action, the bonding strength of the pin is increased.
A printed-wiring substrate formed from resin employs solder, which has a relatively low melting point as compared with brazing filler metal, for bonding a pin. As a result, the bonding strength of the pin tends to become insufficient. According to conventional practice, in order to cope with the problem involved in a printed-wiring substrate formed from resin, a lead pin serving as an input/output terminal has a flange formed at an intermediate position of a shaft portion thereof, and a hole is formed at a pin-bonding portion of the substrate. The lead pin is soldered to the substrate such that an end of the shaft portion thereof is inserted into the hole. However, preparation of the hole decreases space for wiring within the substrate, thereby decreasing the degree of freedom in design. Meanwhile, even in the case of a printed-wiring substrate formed from resin, it is conceivable that the bonding strength of even a nail-shaped pin can be increased considerably by using solder in a sufficient volume and by employing the bonding structure disclosed in the above-mentioned publication. In this case, soldering practice shown in FIG. 6 is considered appropriate. Specifically, a pin 121 is brazed to a pin-bonding portion 111 of a substrate 101 such that a brazing filler metal 131 covers the entirety of a flange 123 as in the case of internal chill (i.e., enveloped casting).
3. Problems Solved by the Invention
However, the above-mentioned practice raises the following problem. When the brazing filler metal 131 is applied so as to cover the entirety of the flange 123, the molten brazing filler metal extends (creeps) by wetting from the root of a shaft portion 122 of the pin 121 toward a tip end of the pin 121 (downward in FIG. 6). This means that the brazing filler metal 131 adheres to at least the root of the shaft portion 122. This impairs appearance and hinders insertion of the pin 121 into a socket of a motherboard after assembly into a semiconductor device, thereby impairing reliability of electrical connection.
As shown in FIG. 7, by employing a bonding structure for the pin 121 which reduces the amount of the brazing filler metal 131 to be consumed for bonding, extension by wetting of the brazing filler metal 131 to the shaft portion 122 of the pin 121 can be prevented. However, a narrow portion K, in the form of a meniscus, of the brazing filler metal 131 is formed. As a result, when an external force is applied to the pin 121, stress concentration tends to occur, thereby impairing the bonding strength of the pin 121 and the reliability of electrical connection. That is, when pin bonding is performed so as not to allow extension by wetting of a brazing filler metal of low melting point to the shaft portion as in the case of brazing with respect to a printed-wiring substrate formed from resin, a failure to obtain a desired bonding strength is highly likely; therefore, strict inspection of pin-bonding strength or quality control must be conducted.
In view of the above-mentioned problem involved in a printed-wiring substrate having lead pins, such as a PGA type printed-wiring substrate, an object of the present invention is to provide a printed-wiring substrate having lead pins brazed to corresponding pin-bonding portions of a substrate in which solder does not adhere to a shaft portion of the lead pin or impair pin-bonding strength, to thereby provide a highly reliable electrical connection.
The above object of the invention is achieved by providing a printed-wiring substrate comprising a substrate having pin-bonding portions formed on a main surface thereof and lead pins, the lead pins each having a flange and a shaft portion and being brazed to corresponding pin-bonding portions via the corresponding flanges, the flange having a convex portion formed on a bonding surface thereof in opposition to the pin-bonding portion, the printed-wiring substrate characterized in that:
a brazing filler metal brazing the lead pin to the pin-bonding portion extends by wetting toward the tip end of the lead pin beyond the outermost edge of a surface of the flange opposite the bonding surface such that the end of an extension of the brazing filler metal formed by wetting is located between the outermost edge and the shaft portion.
In the printed-wiring substrate having lead pins of the present invention, the brazing filler metal which extends by wetting does not reach the shaft portion and thereby does not hinder insertion of the lead pins into a socket. The brazing filler metal extends by wetting toward the tip end of the lead pin beyond the outermost edge of the surface of the flange opposite the bonding surface of the same. Such extension of applied solder means that the amount of applied solder is appropriate so as not to cause insufficient bonding strength.
When a molten brazing filler metal fails to extend beyond the outermost edge of the opposite surface, a narrow portion, in the form of a meniscus, of the brazing filler metal is formed as shown in FIG. 7, or the contour of a section of the brazing filler metal as sectioned by a plane including the axis of the shaft portion of the lead pin assumes the form of a concave arc (hereinafter, the term xe2x80x9cnarrow portionxe2x80x9d includes the form of a concave arc). As a result, when an external force is applied to the lead pin, a brazed (bonded) portion tends to break due to stress concentration. By contrast, when the brazing filler metal extends beyond the outermost edge of the opposite surface as in the case of the present invention, the applied brazing filler metal is free of a narrow portion, because of sufficiency in the amount of applied brazing filler metal, thereby avoiding a substantial impairment in brazing strength. Thus, the present invention can provide a printed-wiring substrate having lead pins which does not raise a problem during insertion of the lead pins into a socket and is free of impairment in brazing strength and which provides a highly reliable electrical connection.
As mentioned above, according to the present invention, no problem arises during insertion of lead pins into a socket, and brazing strength is not impaired. Furthermore, since the lead pins are usually plated with gold, visual inspection can determine whether or not the extension-by-wetting end of molten brazing filler metal is located at the above-mentioned position. That is, the printed-wiring substrate of the present invention significantly facilitates inspection for assurance of quality, or quality control. Herein, the term xe2x80x9cbrazing filler metalxe2x80x9d includes solder, and the term xe2x80x9csolderxe2x80x9d refers to a brazing filler metal having a melting point not higher than 450 degrees.
In a preferred embodiment, the entire bonding surface of the flange assumes a hemispherical form so as to serve as the convex portion. The shape of the convex portion is not particularly limited, so long as a sufficient amount of brazing filler metal is present between a pin-bonding portion of the substrate and the flange of a lead pin. Accordingly, the shape of the convex portion is preferably such that the entire bonding surface tapers off, such as conical or pyramidal. Particularly preferably, the entire bonding surface assumes a hemispherical form, for convenience of manufacture of lead pins.
The entire bonding surface does not necessarily serve as the convex portion. A portion of the bonding surface may serve as the convex portion. In this case, the convex portion does not necessarily taper off, but may assume the form of a circular cylinder or a prism. When a portion of the bonding surface serves as the convex portion, the convex portion is preferably located at the center of a bonding-surface side of the flange.
In the present invention, preferably, the diameter of a brazing surface of the pin-bonding portion is greater than the diameter of the flange. The diameter of a brazing surface of the pin-bonding portion refers to the diameter of the pin-bonding portion when a peripheral region of the pin-bonding portion is not covered with solder resist, or the diameter of an opening portion of solder resist when a peripheral region of the pin-bonding portion is covered with solder resist. In the present invention, preferably, the contour of a section of the brazing filler metal as sectioned by a plane including the axis of the shaft portion of the lead pin is substantially linear, and an angle xcex8 between the contour and the main surface of the printed-wiring substrate falls within the range of 55 to 80 degrees.
The present invention is suitably applied to a printed-wiring substrate made of resin. A PGA type printed-wiring substrate is mentioned above as a typical example of the present invention. However, the present invention is not limited thereto. The printed-wiring substrate of the present invention encompasses a substrate having lead pins brazed to corresponding pin-bonding portions thereof, such as an interposer, which is bonded to a printed-wiring substrate (IC package) so as to connect the printed-wiring substrate to a motherboard.